The present invention relates to a pipette for replaceable pipette tips.
Pipettes are used in the laboratory in particular for metering liquids. For this purpose, a pipette tip is clamped fast on a seat of the pipette with an upper opening. The seat is mostly a conical or cylindrical projection with respect to a casing of the pipette, onto which a pipette tip can be clamped with the upper opening thereof. The pipette tip can pick up and give out liquid through a lower opening. Air cushion pipettes comprise a displacement equipment for air, which is communicatingly connected to the pipette tip through a hole in the seat. An air cushion is relocated by means of the displacement equipment, so that liquid is sucked into the pipette tip and ejected out from there. For this purpose, the displacement equipment has a displacement chamber with a relocatable limit. The displacement equipment is mostly a cylinder with a piston that can be relocated therein.
After use, the pipette tips are released from the seat and replaced by a fresh pipette tip. Contaminations in subsequent meterings can be avoided through this. Pipette tips have usually an ejection device for ejecting the pipette tips, which permit ejection by actuation of a button without having to touch the pipette tips. Single use pipette tips made of plastics are available at low cost.
The relocatable limit is coupled to a drive equipment, which serves for shifting the piston in the cylinder. The drive equipment has a lifting rod, which can be shifted between an upper and a lower stop with a stop element. In the beginning of the aspiration of air into the displacement chamber, the stop element is situated at the lower stop. In the beginning of the displacement of air out of the cylinder, the stop element rests on the upper stop. The amount of liquid that is picked up or delivered, respectively, depends on the stroke of the relocatable limit, and thus on the stroke of the lifting rod. The stroke volume of the relocatable limit does not correspond exactly to the amount of liquid that is picked up or delivered. As the air column expands somewhat under the weight of the liquid, the stroke volume exceeds the volume of the liquid. The deviation between the stroke volume and the liquid's volume depends in particular on the density and viscosity of the liquid, the temperature, the air pressure and on wetting effects. For instance from the document WO 03/0331515 or U.S. Pat. No. 3,827,305, it is known to calibrate pipettes to a certain metering volume by adjusting the position of an upper stop body.
In fixed volume pipettes, the distance between upper and lower stop is constant. A fixed volume pipette with an upper stop body in the form of a threaded sleeve that is adjustable by a calibration tool is known from the document U.S. Pat. No. 4,020,698.
In pipettes with adjustable metering volume, the position of the upper stop is variable. Known pipettes have an upper stop body in the form of a threaded spindle, which is adjustable in a spindle nut which is fixedly disposed in the casing. In order to adjust the threaded spindle, there are adjustment equipments, which are coupled to indicating equipments in the form of a counter for indicating the set metering volume.
The documents DE 43 35 863 C1 and U.S. Pat. No. 5,531,131 describe a pipette where a cylindrical actuating element projects out of the casing at the top, and is connected to an upper end of a lifting rod that is connected to the piston at its lower end. The lifting rod is guided through the upper passage channel of a threaded spindle and the lower passage channel of a lower stop body. It comprises a stop element in the form of an outward projecting bead, which limits the movement of the lifting rod between the threaded spindle and the lower stop body. By pressing in the actuating element against the force of a pull back spring, the piston is moved deeper into the cylinder, until the stop element bears against the lower stop body. After releasing the actuating element, the piston reverts into its starting position due to the action of the pull back spring, in which the stop element bears against the threaded spindle. Adjustment equipments for adjusting the threaded spindle comprise an adjusting sleeve, which projects out of the casing at the top and in which the actuation button can be relocated axially. The adjusting sleeve is rotatably mounted in the casing and connected to the upper end of the threaded spindle via catch dogs so as to rotate together with it. By rotating the adjusting sleeve, the threaded spindle can be relocated together with the spindle nut, wherein the catch dogs are axially relocatable in axial grooves of the adjusting sleeve.
Moreover, pipettes are known in which a cylindrical actuating element serves as an adjusting element for adjusting the threaded spindle at the same time. For this purpose, the actuating element is connected to the upper end of the threaded spindle so as to rotate with it and to be axially relocatable. A driving tenon in the form of a polygon on the upper end of the threaded spindle immerses into a complementary axial accommodation of the actuating element. The actuating element is relocatably arranged in a break-through of a sleeve-shaped transmission part, which is rotatably mounted in the casing. The actuating element is connected to the transmission part so as to be rotationally blocked via catch dogs in the form of ribs that project outward engaging into axial grooves of the transmission part. The transmission part has a toothed ring with axially projecting teeth on its circumference at the outside, into which a toothed wheel of a counter mechanism engages which serves for indicating the set metering volume.
The known pipettes have a locking equipment which prevents that a set metering volume is changed without intention in the metering. For this purpose, an axially directed tooth on a lever arm of a two-arm lever, which is pivotal around a horizontal axis, engages between two neighbouring axially directed teeth of a toothed ring on the circumference of the transmission part. The lever is pressed into this locking position via a spring. In order to unlock, there is a push button which partly projects out of the casing and acts within the casing on the other lever arm of the lever via a chamfered surface. By pressing the push button deeper into the casing, the lever is swung so that the tooth is released from the toothed ring. In this position, the actuating element can be rotated in order to adjust the metering volume. Such a locking of the rotational position of the actuating element is described in the document EP 0 527 170 B1.
The known locking equipment has the disadvantage that it may be destroyed in the locking position by rotating the actuating element with increased force. Moreover, it is tedious to push the unlocking push button and to adjust the metering volume at the same time. Moreover, actuating the wedge gear system formed by the push button and the lever requires a relatively high expenditure of force. Further, the engagement of the tooth into the toothed ring of the transmission part limits the fineness of the setting of the metering volume. Moreover, the assembly of the many single parts is sumptuous.